Even if Nelly had his most successful single in years with 2010's "Just a Dream," his defining numbers go back over a decade before this 2013 release, with "Country Grammar" landing in 2000 and the massive "Hot in Herre" dropping in 2002. The serene and smooth "Just a Dream" was also an odd duck for the party-time rapper as he always seemed more comfortable popping bottles and dropping drawers, so the semi-sweet M.O. splits the difference, dropping "Just a Dream" sequels like the uplifting "Heaven" with Daley, and the less-successful "Headphones" with Nelly Furtado, a rap-by-numbers "we need something that sounds like B.o.B's 'Airplanes'" track. The rapper Nelly's unique voice -- high-pitched but soulful enough not to be strident -- is surprisingly up to the task of these ballads, and he sounds sincere even when the material is unexceptional, but the creative flights of fancy found on his previous album, 5.0, are back too, only this time, they're the highlights. "Rick James" with T.I. is a disco-funk joy fueled by Pharrell Williams in full Neptunes mode with live drums and a slowly rocking beat, but the album cut to cherish is the effortless stunner "Maryland, Massachusetts," where Nelly rattles off "Keep a bad bitch from St. Louis to Vallejo/Your bitches they don't exist, Manti Te'o" and a bunch of other clever stingers. That guests tend to rule their cuts as "Idgaf" is so Pharrell it's almost N.E.R.D., while "100K" sounds like 2 Chainz even before the rapper hits the cut, while Nicki Minaj drops the fantastic common misuse couplet "You should follow my example/Bitch i.e." during the excellent "Get Like Me." Still, that Minaj feature opens the album when it would have been better placed down the track list, and with some redundant numbers to consider, M.O. is both a step up from his previous effort and a mixed bag to rejigger and edit. ~ David Jeffries